1. Field of the Invention
This invention relates to electric current interrupting devices and, more particularly, to a full range current limiting fuse suitable for 23 KV and higher application voltages.
2. Description of the Prior Art
The time current melting characteristics of strap-element current-limiting fuses have always been characterized by a relatively steep, inverse shape. It is known that a current-limiting fuse with a less-inverse, time current characteristic would be desirable and more coordinateable. Experience has shown that wire element expulsion type fuses have the less inverse melting characteristic because of the use of wire.
Some prior art fuses utilize a tin-wire fuse element in series with one or more sections of silver current-limiting strap. This combination results in the desired less-inverse characteristic. However, the fuse is complex because the tin wire is enclosed in a flexible thick-walled silicone rubber tube. The tubes in turn are jacketed with a strong covering of woven fiberglass so that pressure generated by the melting and arcing of tin, during interruption, does not explode the silicone rubber tube which would otherwise nullify their ability to assist in the clearing of low currents. Low current clearance is accomplished in the silicone rubber tube design by virtue of generated pressure within the tube blowing the molten tin out of the tube and the current path and into the relatively cool sand where it condenses near the end of the tube. High current clearance is accomplished in the ordinary manner by the series silver strap elements as in any backup type of current limiting fuse.
Associated with the foregoing is the fact that current-limiting fuses are usually mounted vertically which causes the top of the fuse to operate hotter than the lower end so that melting temperatures are affected. This is particularly true where the tin wire is disposed at the hotter end of the tube which causes it to have variable melting characteristics. That is, with the tin wire at one end the melting characteristic band is widened, thereby resulting in an overly wide band resulting in a less coordinateable device.